In association with the remarkable progress of portable electronic devices, communication devices and the like in recent years, a lithium ion secondary battery having a high energy density is strongly demanded from the standpoint of the economy and the reduction in size and weight of the devices. As a measure for increasing the capacity of the lithium secondary battery of this type, various measures have been investigated including improvement of a positive electrode and a positive electrode active material, improvement of a negative electrode and a negative electrode active material, and the like. The improvement of a negative electrode and a negative electrode active material having been investigated includes the use of silicon (Si) or a silicon compound as a negative electrode active material. Silicon exhibits a theoretical capacity of 4,200 mAh/g, which is far higher than the theoretical capacity, 372 mAh/g, of a carbon material having been subjected to practical use, and thus is greatly expected for reduction in size and increase in capacity of the battery. Furthermore, silicon is capable of forming an alloy with lithium, and thus has excellent characteristics as a negative electrode material that it may not cause internal short-circuit due to the formation of dendrite on charge and discharge.
For example, JP-A-5-074463 describes a lithium secondary battery using single crystal silicon as a support of a negative electrode active material. For imparting conductivity to a negative electrode material, JP-A-2000-243396 describes a technique of mechanically alloying silicon oxide and graphite and then subjecting to a carbonization treatment, and JP-A-2000-215887 describes a technique of covering a surface of silicon particles with a carbon layer by a chemical vapor deposition method. In these ordinary techniques, the conductivity of the negative electrode material may be improved by providing a carbon layer on a surface of silicon particles, but the problematic low cycle characteristic of the silicon negative electrode active material due to the large volume change associated with charge and discharge have not yet been solved.
In the case where silicon is alloyed with lithium, the volume thereof is increased at most approximately 4 times. On repeated charge and discharge, accordingly, it is considered that a large internal strain occurs within the silicon particles and finely pulverizes the silicon particles to deteriorate the cycle characteristics. Various measures have been investigated for solving the problem of the low cycle characteristics of the silicon negative electrode active material. JP-A-2004-335271 describes a technique of providing a negative electrode active material by mechanically alloying silicon with a metal, such as titanium, nickel or copper, in a ball mill. JP-A-2010-244767 describes a technique of providing a negative electrode active material by treating silicon particles and copper particles with a dry attritor. JP-A-2012-113945 describes a technique of providing an aggregated material by pulverizing coarse powder of silicon with a bead mill and subsequently adding copper powder as conductive base powder thereto, followed by applying a shearing force thereto.
However, even by using these techniques, the improvement of the battery characteristics due to the negative electrode active material, such as the battery capacity per unit mass of the negative electrode active material, and the cycle characteristics, has not yet been sufficient, and it is the current situation that a secondary battery using a negative electrode active material using silicon is not used widely.